Cracked insulators are often signs of DETONATION, a bad thing. Garbage gas or funky timing are the most common issues. For timing issues, use two different lights or magnetic sensor to insure the one used originally is at least reasonably close. One national agency did a study a few years ago and found these instruments varied as much as 10 degrees among those tested.

Leaving detonation go on very long will put you in position to buy new pistons.